I believe that even Public Television uses union actors, so they're probably paid scale.

An interesting bit of slightly off-topic trivia: Puppeteers are eligible to join SAG (screen actor's guild) because during Ronald Regan's tenor as president of SAG, he hired Bob Baker's marionettes to do one of his kid's birthday parties. Apparently he was so impressed with the level of craft and talent that he lobbied to include puppeteers in the union!

The "Sesame Street" actors take outside acting jobs to supplement the income...

An example is Roscoe Orman (Gordon). He's made appearances in movies like "F/X" and "Striking Distance" and on TV shows like "Sex And The City" during his many years on the Street.

Another example is Sonia Manzano (Maria). In the 70s, she made appearances in the first "Death Wish" movie and a late-70s exploitation flick called "Firepower". As the book "Sesame Street Unpaved" notes, she gets killed in both.

I believe that the actors are union and most likely make scale. I'm not familiar with SAG rates in the US (they probably have a website where you can look up rates), but union performers in Canada generally make about $600 Cdn (or about $475 US) per 8 hour day. Puppeteers doing manipulation and voice for characters make scale + 50%. The rate also goes up if the producer is doing a "buy-out", paying extra so they don't have to pay the performers royalties later on. The system is probably similar for SAG, but the rates may be slightly higher. Educational television programs sometimes get breaks on union rates because of the low budgets involved.

Many of the Muppet performers (and possibly the Sesame actors too) are under contract and may make more. Kevin Clash is rumoured to make in excess of $1 million per year, presumably some of that comes from Sesame Street's budget.

Most actors take extra work in commercials, theatre, television, teaching, etc. in order to make a living even if they are on a TV series. Only the very top percentage of actors really make a very good living just acting. It's very much an unglamourous hard working job unless you are someone like Tom Cruise. Alot of actors also rely on royalties which are paid each time a TV show or commercial they are in airs on TV.

Roscoe Orman began Sesame Street while he was playing an abusive criminal on the soap opera All My Children. He knew that one had to go and he wisely decided to stay on Sesame Street. Friendly neighbors on children shows tend to last longer than violent bad guys on soaps.